Automatic electronic aircraft flight control or pilot



8 1948- w. J. HARSHBARGER EI'AL 2,457,230

AUTOIATIC ELECTRONIC AIRCRAFT FLIGHT CONTROL OR PILOT 5 sums-shut 1 Filed Aug. 27, 1943 M1. rm

map A. Parr 3- w. J. HARSHBARGER EIAL 2,457,230

MITOIATIC ELECTRONIC AIRCRAFT FLIGHT CONTROL 0R PILOT Filed Aug. 27, 194.3

5 Shoots-Shut 2 1o INVENTORS 1948, w. J. HARSHBARGER EI'AL 2,457,230

AU'I'QMTIC ELECTRONIC AIRCRAFT FLIGHT CONTROL OR PILOT Filed Aug. 27, 1943 5 Sheets-Sheet 3 a4 ga/ TTOR Y5- 8- w. J. HARSHBARGER EI'AL 2,457,230

AUTOIATIC ELECTRONIC AIRCRAFT FLIGHT CONTROL OR PILOT Filed Aug. 27, 1945 5 Sheets- Sheet 4 BY W A ORN Y;

Dec. 28, 1948. w. J. HARSHBARGER EI'AL 2,457,230

AUTOIATIC ELECTRONIC AIRCRAFT FLIGHT CONTROL OR PILOT Filed Aug. 2'1, 1943 5 Shoots-Shoot 5 Patented Dec. 28, 1948 AUTOMATIC nmc'raomc Amcam rucn'r coN'raoL on rmo'r Walter J. Harshbarger, New York, N. Y., and Donald R. Putt, Jersey City, N. 1., auignors to Rex Cole, New York, N. Y.

Application August 27, 1943, Serial No. 500,228

Claims. 1

This invention relates to an automatic electronic aircraft flight control or pilot. More particularly, the invention relates to automatic means for controlling or stabilizing the flight of an airplane, in the operation of which means a deviation from a normal or other desired course or conditions of night is detected and the controls of the airplane actuated automatically in a new and desirable manner to maintain the plane to the desired course or conditions of flight.

The so-called automatic pilots have been known heretofore. In the operation of the conventional automatic pilots in use at this time, a

deviation of the airplane from the desired flight conditions, for example, a dip in one of the wings, is detected by a gyroscope and a corrective force applied to the ailerons to return the wings to the desired level position; but the amount of correction applied to the ailerons is generally proportional to the deviation of the plane from the desired level condition. Thus, if a wing dips only a small amount from a level position, the automatic pilot moves the ailerons only a relatively small amount. If the wing dip is larger, the ailerons will be moved further than if the dip is small. Thus, in accordance with these conventional automatic pilots, the corrective force may be regarded as a function of the magnitude of the deviation from the normal or desired conditions.

The previously known automatic pilots also may be operated so that upon large deviations of the airplane from the normal or desired flight conditions (generally above a. fixed amount), for example, a relatively big dip by one of the wings, the ailerons would be moved to maximum position and held there until the wings had returned almost to a level position whereupon the ailerons would be moved back to their normal position.

These forms of control are apt to cause overcorrection, and so-called "hunting or oscillating, either because the greatest magnitude of the corrective force is not applied to return the plane to the desired or normal condition until the plane has actually been moved to the greatest magnitude by the atmospheric or other disturbance, or because the corrective force is retained too long. If the corrective force applied by the automatic pilots is always proportional to the magnitude of the deviation of the plane, it will be apparent that the corrective force is not removed completely until the plane is in normal position. Inertia and other factors cause the plane to move beyond the normal position, and therefore a corrective force in the other direction must be applied. This continues with a tendency for the plane to hunt or oscillate around the normal position. These conventional systems also have the disadvantage either that the full amount of the corrective force needed is not applied until the plane has actually i deviated to the maximum position from the normal, or else the corrective force is a maximum. In either event there is the tendency for the plane to be subjected to rather large and violent movements and the consequent motion is unpleasant 10 to passengers and possibly damaging to the cargo and plane.

In accordance with this invention, not only the magnitude of the deviation 0! the plane from the normal position is detected, but also the rate of deviation, and the corrective force applied is a function of both the magnitude of deviation and the rate of deviation from the normal. Thus if an atmospheric disturbance is such as to cause one of the wings, for example, to dip a relatively go large amount, the control constituting the invention determines the rate that the plane starts deviating from the normal, as it moves through the first few degrees. Fromthis rate it is determined that if unchecked the plane would eventually deviate, for example, 20. Then, in accordance with the invention, a corrective force corresponding to a 20 deviation or a greater corrective force, may be applied to the ailerons as soon as this detection of rate is made. Thus the ailerons are adjusted, not merely in accordance with the magnitude of deviation at the time the corrective force is supplied, but rather in accordance with the magnitude that would eventually take place as determined by the rate of deviation. The full corrective force is applied at once to prevent the major portion of the deviation, rather than after the deviation has occurred. This mode of operation prevents the plane from deviating very far from the normal position and tends to stabilize the position of the plane so that the movement is of relatively small magnitude.

In accordance with the invention, therefore, the corrective forces are applied both in relation to the magnitude of the deviation and the rate of deviation from the normal. The relative proportion between these two factors may be variable and under the control of the pilot. Thus in accordance with the invention, the deviation in magnitude may be considered to be modified by the rate of this deviation. and the extent of this modification is variable. If the amount by which the rate modifies the magnitude is small, the operation oi the control approaches that of the conventional prior art described heretofore. On the other hand, if the rate is permitted to modify the magnitude to the maximum extent possible in accordance with the invention, the actual ma nitude oi deviation of the plane may be extremely small. Because of different types of atmospheric conditions, diiierent types of airplanes and constructions, and the desires of different pilots, the relation between the correction due to deviation in magnitude and rate of deviation may be subject to manual or automatic control.

The action 01' the automatic control comprising the invention is somewhat like that a pilot uses in flying a plane manually. If the pilot senses that an atmospheric disturbance is of such a nature that it will cause one of the wings to dip 20", he does not wait until the wing has dipped this amount before moving the aileron to the full corrective position to return the wing to normal position. Rather, he detects, through his experience and intuition, that the disturbance is one of considerable force, and immediately moves the aileron to correct or neutralize an atmospheric force of this amount. Thus, he may be able to prevent the plane from leaving the normal position by more than a very few degrees.

In accordance with this invention by detecting the rate of deviation, and by means of suitable electrical circuits to be described hereinafter, it is possible to control the movements of the ailerons and other controlling parts of the airplane, to hold the airplane in a stabilized position of flight and with a minimum of movement.

It will be seen, in view of the above explanation, that it is an object of the invention to control the flight of an aircraft, or stabilize its flight, by detecting the rate of deviation from the normal position, and utilizing this rate as a corrective factor for controlling and/or stabilizing the flight of an airplane.

It is a further object of the invention to control or stabilize the flight of an airplane in such a manner that the movement of the plane as a result oi atmospheric or other disturbances is a minimum, contributing to the comfort of passengers, or the safety of the cargo and plane.

A further object of the invention is the provision of automatic means for controlling or stabilizing the flight of an airplane in which both the magnitude of the deviation and the rate of the deviation are considered and applied for control purposes, and the relative proportion between these two varied at the will oi! the pilot.

Still a further object is the provision of a control, operable in the manner above described, which is electronic in character, has a minimum number of movable parts, is smooth and sure in operation, and readily constructed and installed.

The invention may be further understood by reference to the drawing forming part oi! the specification, and in which:

Figure 1 is a perspective illustration of an airplane, in which the invention is shown as applied to the control of the ailerons for illustrative purposes;

Figures 2, 3 and 4 are to be read together, and illustrate the electrical circuit or a control constructed in accordance with the invention; and

Figure 5 is a diagrammatic illustration of the wave form of alternating currents flowing in parts of the circuit in Figures 2, 3 and 4.

Referring more particularly to Figure 1, an airplane is shown diagrammatically. For the purposes 01' considering stability and motion of the airplane, it may be considered as having three axes. The vertical or Z--Z axis may be considered as perpendicular to the ground when the airplane is in normal flying position. The lateral or transverse axis (YY) is parallel to the ground when the wings of the airplane are level. The longitudinal (X-X) axis is parallel to the ground when the ship is horizontal. When the airplane rotates about the Y-Y axis, the nose goes up or down. This is called pitch," and is controlled by movement of the elevators a. When the airplane rotates about the X-X or horizontal axis, the movement is called roll or blank. This is caused by or controlled by the ailerons b. Movement about the vertical or Z-Z axis results in changing direction in which the nose is pointing, or the direction of travel and is called yaw. This is caused by or controlled by movement of the rudder c.

The invention may be applied to control the movement of the airplane about all three axes or in two of them or any one of them. For illustrative purposes, the invention will be described as applied to the control of movement about the XX or longitudinal axis, and it will be apparent from this description how the invention may be similarly applied to the control 01 movement about either or both of the other two axes.

Mounted in the airplane, preferably near the point through which the three axes pass, is a gyroscope G, or other device such as a pendulum, for indicating the movement of the airplane about the X-X axis. If the gyroscope is so constructed. it may also indicate movement about the Y--Y and Z Z axes. Construction of these gyroscopes is well known, and since they are not a part of the invention, the gyroscope is illustrated diagrammatically. It comprises a rotating member (1, supported in gimbals c, which, in turn, are supported in a fixed support f. Extending from the gyroscope is a shaft 9, which rotates relative to the plane in proportion to the deviation of the plane from the horizontal position about the XX axis. Mounted on the plane is a p'otentiometer I having a movable center tap IA, which is mounted for rotation by the shaft 9. Alternatively the center tap maybe fixed on the plane and the resistance element 1 rotated in acc0rdance with the airplane movements. One segment of the potentiometer i on the right side of the center tap IA may be designated as 113, and the other segment of the potentiometer may be designated as IC. Three wires or electrical conductors i2, 5, and II, respectively, are brought from the potentiometer and their position in the electrical circuit as indicated in Fig. 2.

The ailerons b may be actuated by a so-called servomotor 6! (Fig. 3) mounted within the housing S having a drive pulley m. A flexible cable n is wound around the drive pulley for two or three turns, and carried over suitable pulleys to operate the ailerons b through a leverage arrangement p. This construction is well known in the art, and since it, forms no part of the invention, it is described and illustrated diagrammatically.

The servomotor may have two windings, so that the pulley m may be rotated in either direction. These windings may be selectively energized by means of relays, as will be described more fully later. The form of servomotor is not particularly critical and any form may be used which will drive the pulley m in either of two directions upon transmission to it of either of two of the electrical impulses. Mounted on the servomotor is a small shaft q, which rotates the same amount and in the same direction as the drive pulley 112. Fixed on the airplane is a potentiometer 2, having a movable center tap 2A, which is mounted for movement by the shaft q. Thus, as the servomotor moves in either direction, the center tap 2A will be moved along the resistance element of the potentiometer 2. The portion of the potentiometer on the right side of the center tap is indicated as 23, and the portion of the potentiometer on the left side of the center tap is indicated as 20. Wires or conductors I2, 4 and II, respectively, are brought from the potentiometer 2 and their position in the electrical diagram is shown in Fig. 2.

When the airplane is flying with the wings level, the center tap IA of the potentiometer I and the center tap 2A of the potentiometer 2 are in the center of the resistance elements, respectively of the potentiometers I and 2, as shown. If a potential is impressed across the potentiometers, through wires II and I2, the potentials at the center taps, as reflected through wires 4 and 5, respectively, will be the same. If the plane banks in either direction, this will be detected by the gyro, the center tap IA will move to either sections IB or IC of the potentiometer I and there will be a difference in voltage between wires 4 and 5. The magnitude of this voltage will vary directly in proportion with the deviation of the plane from a level position. By means of the invention this voltage is modified in accordance with the rate at which'the voltage is developed, as described heretofore, and then amplified. The modified and amplified voltage is then used to operate the servomotor, which moves the ailerons b in the proper direction and in the amount to compensate for the deviation of the airplane, taking into account both the magnitude of the deviation and the rate of deviation, as determined by the voltage, and the rate of change in the voltage, respectively, between the conductors 4 and 5.

As the corrective force exerted by the ailerons b returns the plane to the normal position, the difference in potential between the conductors 4 and 5, depending upon the magnitude, and the rate at which the voltage is developed causes the servomotor to continuously adjust the ailerons in accordance with the method described generally heretofore, so that the airplane will be prevented from overrunning the normal position. This mode of operation prevents hunting and also prevents the airplane from deviating to any appreciable extent from the normal position; or if there is a deviation, it will be caused to return to the level position. When the difference in potential between the conductors 4 and 5 disappears, the plane will proceed in its normal and level course.

It will be apparent that the elevators a, or the rudder c, and both of them may be controlled in the same manner, and such control is within the invention, but a repetitious explanation is not believed necessary.

The manner in which the difference in potential between the conductors 4 and 5 in relation to the potential applied across the potentiometers I and 2 is used to achieve the control of the type described will be understood from a consideration of'Figs. 2, 3 and 4.

In these figures, three separate channels are shown, operated by a common power supply; one channel is utilized to control the movement of the ailerons b, the second channel is utilized to control the movement of the elevators a, and the other channel utilized to control the movement of the rudder c. The operation of only one chan- 6 nel will be described in detail, namely, that used to control the operation of the ailerons, since it will be obvious to one skilled in the art, in view or this description, how the other two channels may be used to control the operation of the elevators and rudder, respectively;

In the upper left-hand portion of Fig. 2 the potentiometers I and 2 referred to in Fig. l, are illustrated together with the center taps IA and 2A. The outer ends or the segments IB and 2B of the potentiometers are each grounded at II and an A. C. potential is supplied through the wire I2 from the secondary I3 of the transformer II! (Fig.4).

The center tap 2A of the potentiometer 2 is connected through wire 4 to the grid of a triode 3. A center tap IA of the potentiometer I is connected through wire 5 to the cathode of the triode 3. Thus a difference in potential between the wires 4 ant 5, which is developed when the plane starts to bank, is impressed between the cathode and grid of the tube 3 and the amplified voltage developed in the plate circuit is in proportion to the magnitude of the deviation of the plane from horizontal. The plate voltage for the tube 3 is supplied from the plate supply line 15, through the plate resistance 29, wire 30, resistance 3| and wire 32. The A. C. output of the tube 3 passes through wire'32, resistance 3|, wire 33, condenser 34 and wire 35, and is impresed upon the grids of the tubes 36 and 31. The latter two tubes operate the relays 46 and 56 which in turn control the operation of the servomotor 6I.

Upon actuation of relay 46 contacts 53 and 54 are closed which causes the servomotor 6| -to drive in one direction by passing current from the battery 55 through the wires 56, 51, contacts 54 and 53, wire 56, motor field '59, wire 60, armature 6I to ground I I. The closing of contacts 53 and 54 may also serve to operate clutches of electric or hydraulic servomotors, or valves of hydraulic motors, or other similar motive means. The relay 46 has a second set of contacts 61 and 61A which are also closed upon actuation of the relay for a purpose to be described hereinafter.

Upon actuation of relay contacts 63 and 64 are closed which causes the servomotor 6| to drive in the other direction by passing current from the battery through the wires 56, 62, contacts 64 and 63, wire 65, motor field 66, wire 66, armature 6I to ground II. The relay 46 has a second set of contacts 68 and 68A which are closed upon actuation of the relay for a purpose to be described hereinafter.

The actuation of the servomotor in response to the voltage delivered from the tube 3 will be in proportion to the voltage differential between the center taps IA and 2A and thus directly in pro portion to the magnitude of deviation of the plane from the normal.

There will now be described the portion of the circuit which relates to the control based upon the rate of deviation and the manner in which this modifies the control based upon the magnitude of deviation.

The center tap IA of the gyroscope potentiometer I is connected through the wire 11 to one end of a gain control potentiometer I8 having a center tap 18A; the other end I9 of the potentiometer I8 is grounded. Thus the potential between the center tap IA of the gyroscope potentiometer I and ground is impressed across the potentiometer I8. The center tap 18A is connected to the grid of the triode 80. By means of the potentiometer I8 and the center tap "A, which is manually variable by the pilot, any proportion of the potential between the center tap IA and ground may be impressed upon the grid of the triode 88, depending upon the position of the center tap "A upon the resistance I8.

The tube III is self-biased by means of the resistance Ma and the condenser Mb. The plate voltage for the tube 80 is supplied from the plate voltage supply line I5 through the primary 83 of the transformer 82.

The output of the tube 80 is impressed across the primary 83 of the transformer 82. The output of the secondary 84 of the transformer 82 is fed to two diodes 85 and 86 and rectified thereby. The rectified output is filtered by means of resistors 81, 88, 98 and condenser 89. The rectified and filtered output is then applied across the condenser 9|. As the magnitude of the D. C. voltage impressed across the condenser ill increases or decreases, the condenser 92 will accordingly charge or discharge through the recti fiers 93, 84, 95, 96 and current will fiow through the resistor 91. The magnitude of the voltage appearing across the resistor 91 will be a function the current fiowing through it, which current is developed by the charge or discharge of the condenser 92. Inasmuch as the charge or discharge of the condenser 92 is dependent upon a change of conditions and introduces a time factor, the voltage impressed across the resistor 91 will be a function of the rate of change of the A. C. voltage apearing between the center tap IA of the gyroscope potentiometer I and ground II.

Center tap 98 in the potentiometer resistance 81 is connected to the grid of the triode I5. The cathode is connected through wire Hi to the neg-- ative or C bias supply line 20. The plate of the tube 15 is connected through wire I4 to the grid of tube I3. The cathode of tube I3 is connected to ground and the plate of the tube I3 is connected to the wire 33.

Current fiows through the tube I from ground II, resistor 69, wire 1|, resistor I2, wire I4, plate to cathode of tube 15, wire I6, and wire 28 to This plate current biases grid bias rectifier 45. the grid of the tube I3 below cut-ofi except when the rate of change of the A. C. voltage between the center tap IA of the gyroscope potentiometer and ground exceeds a given rate. When the voltage impressed on the center tap 98 exceeds the given value, the plate current in tube I5 decreases, lowering the negative grid bias of tube II to such an extent that the positive cycles of the A. C. voltage impressed across resistor 69 causes plate current to fiow in tube I3 during these positive cycles. Thus the plate of tube 13 passes current and its A. C. output voltage is then added to the A. C. output of tube 3 when the A. C. exciting voltage and the output voltage 01' tube I5 exceed the cut-oi! point. The resistor 91 and the center tap 88 provide adjustment for varying the operating point at which the tube ll allows the tube I3 to pass current. This can also be varied by changing the value of the resistor 81 by means of the contact 81a.

The A. C. exciting voltage developed across the resistor 68 and applied to the grid of tube I3, as described in the previous paragraph, is supplied through wire II and is applied to the grid 0! the tube I8 by the closing of either relay 48 and associated contacts 8! and 61A, or relay 50 and associated contacts 68 and 68A. When the relay 48 is closed the circuit may be traced from one side of secondary I3 of the transformer II! through wire III, contacts 81A, 81, resistance I8, wire II, resistance I2 to the grid of the tube I8. When the relay to is closed the exciting voltage applied to the grid oi the tube I3 will be of the opposite polarity and is obtained from the other half of the secondary I3 of the transformer II2, through wires I2, Ila, contacts 68A, 68, resistance IIIA, wires IIA, II and resistance I2 to the grid of tube I3.

When the rate of change of the voltage between center tap IA and ground is large enough so that the tube I3 is biased above cut-oil, the A. C. output of the tube 13 is fed into wire 33 and added to the output of the tube 3. This in effect expands the A. C. voltage developed by the tube 3, which expanded voltage is applied to the grids of the tubes 38 and 31.

When the rate of dip of the Wings of the airplane is relatively great, and the rate of voltage change between center tap IA and ground is also relatively great, the voltage developed across resistor 91 will be relatively great, and the voltage added to the output of tube 3 will be relatively great. Thus the greater the rate of deviation, the greater will be the movement of the servomotor as controlled by the tubes 38 and 31 and the relays 45 and 50.

The extent to which the output 01 tube 3 is modified by the tubes and 73 may be controlled by the potentiometer I8 as explained previously.

Reference has been made previously to the plate voltage supply line I5 and the negative or C bias voltage supply line 20. These voltages are provided by a power transformer I I2 and an appropriate rectifier.

The power transformer I I2 has a primary winding II3 which is connected to the airplane's A. C. power supply, and a secondary winding I4 the center of which is grounded at I I. The output of the secondary I4 is carried through wires I8 and I9, to wires H and 22, and to the plates ISA and I6B oi the rectifier tube IS. The cathode IGC of the tube I6 is connected to the wire 23 which leads to a filter generally indicated within dotted lines as IT. The filter includes chokes 24, 25, condensers 2G, 21 and 28 and a resistor 2|. The filter output is supplied to the line I5 as shown at the bottom of Fig. 4, and the plate voltage supply appears between wire I5 and ground I I.

The negative or C bias voltage supply, which appears between the ground II and wire 20 is also provided by the secondary I4 of the transformer H2. The circuit supplying the C bias voltage may be traced from the ground II, resistance 43A, plates 45A and 45B, cathodes 45C and 45D of the tube 45. resistors 4| and 42, wires I8 and I9 to the center tap of the winding I4 of the transformer II2. A voltage regulator tube 44 is provided between the plates 45A and 45B and ground I I to maintain the C bias voltage substantially constant; a condenser 43 between the cathodes 45A, 45B and ground serves as a filter.

The (3" bias for tubes 38 and 31 is obtained by the drop across the resistance 38 from wire 28 to ground II. The amount of C" bias may be varied by the center tap 38A and may be traced from 38A, resistances 38, 40 and wire 38 to the gridsof tubes 38 and 81.

The secondary I4 of the transformer III through wires I8 and I9 also supplies the voltage for the operation or the relays 46 and 80 and the plate voltage for tubes 36 and 3 1. These circuits may be traced as follows: For the tube 38 and the I relay 46 the voltage appears from the winding I4, wire I6, through the winding on the relay 46 and condenser 41, re 43. to the plate of the tube 36, through the t be to wire 43, and ground I I. The circuit to the tube 31 and relay 60 may be traced from the winding I4, wire I3. winding of the relay 60, and condenser SI, wire 62, to the plate of the tube 31, through the tube 31 to wire 40 and ground II. The condensers 41 and 6| are used to avoid chatter of the relays because they are operated by A. C. current.

Since the voltage developed in the winding I4, and supplied over the wires I8 and I9, is alternating, the polarity in the windings of the relays 46 and 50 and the polarity of the voltage applied to plates of the tubes 36 and 31 in respect to the polarity of the voltage applied to the grids of the tubes 36 and 31, will determine which tube passes current and which one of the relays 46 or 60 will be operated.

The power transformer I I2 also includes a secondary winding Ia which terminates in wires IM and I02 which operate a multi-vibrator circuit including tubes I03, I04 (Fig. 4). The multivibrator tubes I03 and I04 provide a pulse voltage which appears over wire H4. The frequency of the pulse is adjusted to a subharmonic of the frequency of the A. C. power supply. For example, if the power supply is 60 c. p. s., the pulse maybe c. p. s. The tubes I03 and I04 therefore act as a frequency divider whose output frequency is in a direct phase relationship with the line frequency but is a sub-multiple thereof.

The multi-vibrator synchronizing circuits may be traced from the winding I00a, over wires IOI, I02, plate resistors I05, I06, to the plates of the multi-vibrator tubes I03, I04, respectively, through these tubes to the wire 20, rectifier tube 45, resistors 4I and 42, to the ground II through the center tap of the winding I4 oi the transformer H2. The multi-vibrator includes grid resistors I01, I08, coupling condensers I03, H0 and a condenser III for shaping the outline of a pulse provided by the multi-vibrator.

The output of the 'multi-vibratory tubes I03 and I04 appears between the wire 4 and wire 20. The wire H4 is connected to a grid of a triode I00 and the wire is connected to the cathode of tube I00. The pulse provided by the multi-vibrator is therefore impressed upon the buffer tube I 00. The output of the tube I00 is fed into one end of each of the resistors and 40. The outer end of the resistor 33 is connected ill) to the center tap of the variable resistor 38 which is connected across the wire 20 and ground II. The other end of the resistor 40 is connected to the, grids of the tubes 36 and 31 through the wire 35. The output of the tube I00 therefore biases the grids of the tubes 36 and 31 periodically in accordance with the frequency pulse provided by the multi-vibrator and in this way the operating point of the relays 46 and 50 .is periodically changed. This circuit provides an anti-hunting control for the servo motor, and also functions as a vernier slow-down device and allows the servo motor to respond to very small deviatio as detected by the gyroscope.

The operation of the multi-vibrator and the manner in which the tube I00 controls the operation of the relays 46 and 50 through the tubes 36 and 31 may be further understood from a consideration of Fig. 5 which is a diagrammatic illustration of the wave form of certain of the alternating currents flowing in parts of the circuit and the voltages applied thereto. The alternatinc pulse supplied by the multi-vibrator tubes I03 and I04 through the wire II4 has the form indicated at 20I. 'This is a relatively square wave in view of the constants of the multi-vibrator circuit, particularly the condenser III. The voltage applied to the plate of the tube 36 is indicated at 202 and the voltage applied to the plate of the tube 31 is indicated at 203. It will be seen that the polarities of the voltage applied to the tubes 36 and 31 are always opposite in phase and that th efrequency of the pluse in the wire II4 may be a sub-multiple of the frequency applied to the plates of tubes 36 and 31. At the beginning of a cycle of the pulse, as at the points indicated by 20Ia, 202a, 203a, the voltage applied to the plate of the tube 36 starts to become positive and the-woltage applied to the plate of tube 31 starts to become negative. The voltage applied to the tubes goes through two complete cycles while the pulse voltage remains positive and through two additional complete cycles while the pulse remains negative.

The line 204 indicates the maximum voltage is, when voltages of this amount or less are applied to the grids of tubes 36 or 31 the current passing through the tube and relay is not sumcient to hold the relay contacts closed. The line 205 indicates the minimum voltage that may be applied to the grids of tubes 36 and 31 and have the relays 46 and 60 actuated, that is, when voltages of this amount or greater are applied to the grids of tubes 36 and 31, the current flowing through the tubes and relays is sufficient to close the relay contacts.

The line 206 represents the pulse bias voltage normally applied to the grids of the tubes 36 and 31 which is less than that which causes the relays to operate. The voltage (C), which normally is applied to the grids of tubes 36 and 31, is determined by the voltage drop in resistor 30 in the plate circuit of bufler tube I 00. This may be varied by adjusting the resistor 30. Since the voltage (C) may be increased or decreased, this affords a control for the difference between normal plate voltage 206 and the relay drop-out voltage 204 (the diiference being represented by D). It will be seen that by means of the variable resistor 33 which varies the distanze D between the voltages 204 and 206, the sensitivity of the control may be varied. Thus when D becomes greater the sensitivity is reduced.

The A. C. voltage applied to the plates of the tubes 36 and 31 is indicated at 201. However, since no A. C. grid bias is impressed upon the grids of either tubes 36 or 31, these tubes will be biased at the voltage indicated by line 204.

When an A. C. voltage, indicated at 206, is impressed upon the grids of the tubes 36 and 31, the

tube 36 passes two cycles of plate current to the relay 46 \for every cycle of the pulse current 20I, the other two cycles are being eliminated as shown at 209. When the polarity of the A. C. voltage on the grids of the tubes 36 and 31 is reversed, as shown at 2| 0, the tube 31 will pass two cycles of plate current foreach cycle of the pulse current 20I; the other two cycles will be dropped out as indicated at 2I I. It will be seen that either the relay 46 or 60, depending upon the polarity of the grid voltage, is energized and deenergized continuously when the deviations indicated by the gyroscope potentiometer are small and when the voltage developed from the tube 3 are impressed upon the grids of the tubes II and II are small. This permits small pulses of current to flow through the fields II or II of the servo motor and gives very accurate control thereof.

When the deviations of the airplane are larger, or when the rate of change is larger so that the voltage developed by the tube I as modified by the tube II is larger, the voltage applied to the grids of the tubes II and I1 is larger and fills the trough of the pulse voltage as indicated at III so that all of the current is passed through the tube II or II for every cycle (as distinguished from every two being dropped out) so that the relay is energized continuously and does not make and break the relay contacts intermittently.

The method of operation will now be described for illustrative purposes. Let it be assumed that the right wing (facing the plane, Figure 1) drops slightly and not at an excessive rate. The center tap IA on the gyroscope potentiometer, I will move into the potentiometer segment IC, and an A. C. voltage will appear between wires I and I and the grid and cathode of the tube I. If at that instant the A. C. voltage in the wire I! is in the positive part of the cycle, the grid of the tube I would be negative, thereby reducing the plate current of the tube I. This causes the plate coupling condenser II to become charged as a result of current flowing from the wire II through plate resistor 28 through wires II, II, condenser II, resistors II. II. II to ground II. This charging current also lowers the negative grid bias supplied to the grids of the tubes II and I1, since it alters the drop across the portion of the resistor II which provides the "C bias.

At the same instant the A. C. voltage in the wire II is also in the positive part of the cycle, and as a result the plate voltage of tube II will be positive. As a result of the lowering of the negative grid bias on the grid of tube II and the positive plate voltage applied thereto, current will flow through tube II and through relay II to energize it. Since at the same instant the plate voltage applied to tube I1, and at the same instant the voltage in the wire II is in the negative part of the cycle. the plate voltage of tube II will be negative. As a result, no current will flow through tube II and the relay II will remain open.

Relay II, upon being energized by the flow of current through tube II. closes contacts II and II causing the servomotor II to be operated in one direction by the flow of current from the battery II through the ileld II, moving the ailerons to a position so as to cause the right wing to be lifted. As the servomotor rotates. the center tap 2A of the servomotor potentiometer I is driven towards the segment 20. When the voltage between the center tap IA and ground becomes equal to the voltage between center tap IA and ground. no differential A. C. voltage will appear between the grid and cathode of tube I and no current will flow through condenser II. The voltage applied to the grid of tube II will therefore be that of its normal biased constant. This will lower the plate current of tube II and the relay II will be deenergized. Contacts II and II will open and the servomotor II will stop. Since the ailerons have been moved by the servomotor II, a lifting force will be applied causing the right wing to rise.

It is assumed in this example that the rate at which the wing is raised is not excessive. As the right wing rises, the gyroscope potentiometer 12 center tap IA will be moved toward its normal position, and the potential of the center tap IA above ground will be lower than the potential of the center tap 2A above ground. This results in a positive voltage being applied to the grid of the tube 3, and the plate current of tube I will increase. Condenser II discharges through tube I to ground and from ground through resistors II. II and III to the condenser. Because of this flow of current through a portion of the resistor II, the negative bias on the grid of tube II is increased and thus relay II is prevented from operating.

However, when the line voltage changes its polarity in the next half of the cycle, the voltage applied to wire I! will be negative and the grid of tube I will become positive. Thus during the second part of the cycle, the condenser II will discharge so as to lower the negative bias on the .grids of tubes II and I1 due to the flow of current through a part of the resistance II. Also since during this part of the cycle wire II is positive current flows from wire II through relay winding II, wire I2, tube II, wire 49 to ground I I, relay I0 therefore becomes energized and contacts II and II are closed causing current to how through the field II of the servomotor II driving it in an opposite direction.

The servomotor II continues to rotate so as to move the ailerons to a position to reduce the lift being applied to the right wing. This action will continue until the right wing arrives at its normal or level position at which the center tap IA of the gyroscope potentiometer and center tap 2A of the servomotor potentiometer will both be at their normal or center position.

In this assumed example the rate at which the right wing dips is not excessive so that the voltage developed across the resistor I1 is less than that which would permit current to flow through the tube II and as a result the output of the tube I would not be expanded by the output of tube II.

When either of the relays II or II are energized as explained heretofore, the contacts I1 and IIA and II and IIA, respectively, are closed. However, because the rate of deviation of the wing was not excessive, the circuit created by the closing of these contacts is not such as to alter the operation of the servomotor. Upon closing of contacts I1 and "A, current would be permitted to how from the wire II through resistance II and II to ground. Current may also flow from the wire II through resistance II, tube II to the C bias supply line II. However, since the rate at which the right wing dipped was not excessive and the drop across resistor II is low, the plate current of tube II is maximum which biases the tube II to cut-off. The voltage drop across resistors II and I2 supplies the negative cut-off voltage for the grid of tube II. As a result there is no output voltage from tube 'II to expand the voltage of tube I.

Similarly when relay II is energized contacts II and IIA are closed permitting current to flow from wire I: through resistor IIA, wire IIA, II, resistor II toground II, and from wire II through resistor I2, tube II to wire II. Since, however, the rate at which the right wing was returned was not excessive, as assumed heretofore, the tube II would remain at cut-off and the voltage output of tube I would not be expanded by tube II as explained heretofore.

In the previous description of the operation of the control, it has been assumed that the rate of deviations of the wings from the normal attitude or level position did not exceed a given rent of tube decreases.

assmso 13 predetermined value which is determined by the setting of the center tap "A on the resistor oi the potentiometer II and the center tap II on the resistor 91. In the illustrative example of the invention now to be described, it is to be assumed that the right wing has dropped at a rate exceeding the given determined value. For

' example, a weather disturbance exerting a great pressure on the-right wing may be of such a at which the wing starts to drop is an indication of the amount of force applied to it, and the position to which the wing ultimately may be expected to move. 7

when such a large force is applied, the right wing drops and the center tap IA of the gyroscope potentiometer moves towards the segment IC. The grid of the tube I becomes negative'which in turn would cause the grid of the tube II to become positive, causing the relay 4. to operate as described heretofore. This causes the servomotor II to drive the aileron in a direction to lift the right wing.

Under such conditions the A. C. voltage across the segment in of the gyroscope potentiometer and ground also is increased; the voltage drop rightwinghasstartedtodipasaresultoia force which will eventually cause it to move By the time it has reached 2, for example, the output of tube 9 alone would cause the servomotor to drive the ailerons to a point where the center tap 2A or the servomotor potentiometer is equal to the 2 position of the gyroscope potentiometer center tap IA. However, because of the expansion voltage injected by the tube I3 into the output of tube 3 and onto the grids of tube It and 31 the relay 6 will'be held energized by this higher voltage and cause the servomotor to continue to rotate the ailerons past the halcurrent and become charged. This charging curresistance 91 is a function of the rate of change of the A. C. voltage appearing across the segment 1B of the gyroscope potentiometer.

l The voltage developed across the resistance 91 is impressed upon the center tap .99; the grid of the tube I! is biased negative and the plate cur- The grid of tube 13 is normally biased below cut-off because of the drop across resistors 69 and 12. However when the voltage developed across the resistance 91 lowers the plate current through the tube IS the voltage drop across resistor 69 and I2 decreases and the grid of the tube 13 is raised above cutoii and the tube passes plate current.

The polarity of the A. C. voltage across the ressitors II, 69 and 12 as a result of the closing of contacts 61 and 61A by the actuation of the relay 46 is such as to make the grid of the tube I3 in-the same phase as the grid of the tube 3. This A. C. voltage, when the grid of the tube 13 biased above cut-oil, modulates the plate current in such a manner as to expand the A. C. voltage out-put of the tube 3.

Assume for the sake of description that the anced position. This aileron overdrive will continue until the voltage across the resistor 91 decays through the condensers. Thus while the airplane has moved only through 2, the movement 0! the aileron is greatly past the balanced position and counteracts the weather force applied to the right wing before the wing had an opportunity to move through the greater distance that it otherwise would.

As the voltage across the resistance 91 decays the tube 13 is biased to cut-oil? and the expansion voltage is withdrawn from the output of the tube '3. The servomotor and aileron position is now beyond the balanced position and the A. C. voltage of the opposite polarity is now impressed upon the grid of the tube '3. The polarity of the output voltage of the tube 9 is reversed and relay 49 is deenergized. As soon as the next half of the A C.

cycle occurs, relay 50 is energized, causing the servomotor 6| to reverse direction and the aileron is driven back to a balanced position with the gyroscope. The aileron is not moved to its normal position but only to that which balances the position of the plane as indicated by the gyroscope. The lift therefore is still supplied to raise the right wing. As the wing returns to normal attitude, the center tap IA of the gyroscope potentiometer will also move to the center position and the servomotor and aileron will return to their normal position in the manner described previously. 7

Under some circumstances the wing may move at a rate exceeding the predetermined value as it is returning to the level position. For example, the weather force acting upon the right wing may be suddenly withdrawn and the aileron may be in an advanced position tending to raise the right wing. If the rate of return 01' the right wing to level position is excessive, the A, C. voltage across the gyroscope segment IB of the gyroscope potentiometer would collapse at a rate sufllcient to cause the condenser 92 to discharge through rectifler 93, wire 29, resistor 91, wire 99, rectifier 99, again impressing a negative bias upon the tube 15 which would raise the grid of tube 13 above cut-on in the manner described previously. The relay 50 is now energized and through closed contacts 68 and 68A on A. C. voltage is applied from wire l2, through resistor 10A over wire ll, through resistor 12 to the grid of tube 13. The output voltage of tube 13 expands the output voltage of tube 3 thus holding the relay I0 energized. The servomotor continues to drive the aileron towards or beyond normal position until movement of the wing is accelerated, at which time voltage across the resistor 91 will have decayed, removing the expansion voltage applied by the tube 13. The servomotor will then continue to drive the aileron until the wing is in normal or level position and the center taps IA and 2A of the potentiometer are balanced, whereupon the relay 5!! will be deenergized.

The invention has been described more particularly for illustrative purposes in controlling the movement of the wings about the XX axis. The invention contemplates also the control of the elevators a by a servomotor SI and of the rudder c by a servomotor 8|". These are controlled through gyroscope potentiometers l' and I" respectively and servomotor potentiometers 2' and 2" respectively, The circuits are similar, employing tubes 3', I5, I3, I00, 36', 31', I", 80'', 15'', 13'', I00, 36", 31", respectively, to control theoperatlon of relays 46', 50' and 48'', 50", respectively. These circuits may be operated from the same power source and from the same voltage supply wires in, I2, l5, I8, I9, 20 and Ill. It is believed unnecessary to repeat in detail the description and operation of the circuits for the control of the elevators and the rudder.

It will be apparent that the invention may be applied to control the movements of the plane about any one, two or three axes. For example, the invention may be applied only to the control of the rudder to keep the ship upon its proper course. Alternatively, it may be applied to control only the ailerons and the elevators if the course is to be determined by the pilot.

When the invention is to be applied to the control of the plane about more than one axis, for example, control of the rudder and ailerons, the operation of these two may be somewhat dependent upon each other. Thus the movement of the rudder to alter the course may require movement of the ailerons so that the ship may bank properly as it makes a turn. The modifica tion or interrelation of the voltages may be made at any point and may take place eitherbefore the voltages are applied to the potentiometer at I or I"; or after they are fed into the control, for example, by mixing of the outputs from the tubes 3 and 3' before or after expansion by the tubes II and 13'.

The invention resides in a novel method of control as well as the preferred apparatus disclosed, and it is apparent that any number of different apparatus may be used. For example, if it is desired to control hydraulically actuated servomotors, it is necessary only that the controlling elements of the airplane be actuated in response to and in proportion to the output voltage of the tubes 3, 3, and 3" as expanded by the tubes 13, I3 and 13". It will, also be obvious that many modifications and electrical equivalents will be apparent to those skilled in the art in view of the explanation of the method herein. We intend all of the same to be included as within the scope of the following claims.

We claim:

' L'In a device-for controlling and stabilizing an airplane, means for expressing the magnitude of movement of the airplane about at least one of its axes in terms of a voltage, a circuit including a condenser, variable means for dividing said voltage, means for impressing a divided part of said voltage into said circuit to develop a second voltage as a function of the rate of change of the first mentioned voltage, and means for impressing said second voltage upon said first voltage to develop a controlling voltage which can be used in controlling the airplane.

2. In a device for controlling and stabilizing an airplane, means for expressing the magnitude of movement of the airplane about at least one of its axes in terms of a voltage which is a function of said magnitude, a circuit including a condenser into which said voltage is impressed to develop a voltage which is a function of the rate of deviation, means for developing a voltage as a function of said rate voltage and means for impressing said developed voltage upon said first voltage, when said developed voltage is above a predetermined amount to result in a controlling voltage which can be used in controlling the airplane.

3. In a device for controlling and stabilizing an airplane, means for expressing the magnitude of movement of the airplane about at least one of its axes in terms of an alternating current volt age which is a function of said magnitude, means for rectifying said voltage, a circuit including a condenser into which said rectified voltage is impressed to develop a direct current voltage which is a function of the rate of deviation, means for developing an alternating current voltage as a function of said rectified voltage when it exceeds a given value and means for impressing said developed voltage upon said first voltage to form a controlling voltage which can be used in controlling the airplane.

4. In a device for controlling and stabilizing an airplane, means for expressing the magnitude of movement of the airplane about at least one of its axes in terms of an alternating current voltage which is a function of said magnitude, means for rectifying said voltage, a circuit including a condenser into which said rectified voltage i impressedto develop a direct current voltage which is a function of the rate of deviation, means for developing an alternating current voltage as a function of said rectified voltage when it exceeds a given value, means for varying said given value and means for impressing said developed voltage upon said first voltage to form a controlling voltage which can be used in controlling the airplane.

5. In a device for controlling and stabilizing an airplane equipped with a gyroscope and with a servomotor for operating the controls of the airplane, means for controlling the operation of the servomotor in accordance with variations in a controlling voltage comprising in combination a potentiometer connected to said gyroscope, means for impressing an alternating current voltage across said potentiometer, a potentiometer connected to said servomotor, means for impressing an alternating current across said latter potentiometer, whereby movement of the airplane or movement of the servomotor will result in a difference in alternating current potential between the two potentiometers. means for amplifying this potential, means for rectifying at least a portion of this potential, a condenser and a resistor in series across the rectified potential whereby a direct current potential is developed across said resistor which is a function of the rate of change of said first mentioned potential, means for developing an alternating current potential as a function of said direct current potential, means for amplifying said developed alternating current potential, means for combining said two amplified potentials to develop the controlling potential for use in the operation of the controlling means.

. 6. In a device for controlling and stabilizing an airplane equipped with a gyroscope and a servomotor for operating the controls of the airplane, means for controlling the operation of the servomotor in accordance with variations in a controlling voltage comprising in combinations potentiometer connected to said gyroscope, means for impressing an alternating current voltage across said potentiometer, a potentiometer connected to said servomotor, means for impressing an alternating current voltage acros said second menassmao tioned potentiometer, whereby movement of the airplane or movement of the servomotor will result in a difference in the alternating current potential between the two potentiometers, means for amplifying this potential, means for rectifying this potential, two condensers in paral1e1 and a resistor in series therewith, means for impressing the rectified potential across the condensers whereby a potential is developed across said resistor which is a function of the rate of change of said first mentioned potential, means for developing an alternating current potential as a func tion of said direct current potential when the latter exceeds a given value, means for amplifying said developed potential, and means for combining said two amplified potentials, whereby controlling voitage is developed that may be used to control the operation of said controlling means.

'I. In a device for controlling and stabilizing an airplane equipped with a gyroscope and a servomotor for operating the controls of the airplane, means for controlling the operation of the servomotor in accordance with variations in a controlling voltage comprising in combination a potentiometer connected to said gyroscope, means for impressing an alternating current voltage across said gyroscope, a potentiometer connected to said servomotor, means for impressing an alternating current voltage across said second mentioned potentiometer, whereby movement of the airplane or movement of the servomotor will result in a difference in the alternating current potential between the two potentiometers, means for amplifying this potential, means for rectifying this potential, two condensers in parallel and a resistor in series therewith, means for impressing the rectified potential across the condensers whereby a potential is developed across said resistor which is a function of the rate of change of said first mentioned potential, means for developing an alternating current potential as a function of said direct current potential when the latter exceeds a given value, means for amplifying said developed potential, means for combining said two amplified potentials, a power tube operable by said combined potentials, and a relay operable by the output of said power tube for controlling the operation of said servomotor.

8. In a device for contro ling and stabilizing an airplane equipped with means for detecting movement of the airplane about at least one of its axes and with means for operatin the controls of the airplane in accordance with the effect produced by said detecting means, means for operating. said controls in accordance with variations in a controlling voltage comprising in combination means for expressing the magnitude of movement of the airplane about at least one of its axes in terms of an alternating current voltage which is a function of said magnitude. means for rectifying said. voltage, a circuit including a condenser into which said rectified voltage is impressed to deve op a direct current voltage which is a function of the rate of deviation, two pairs of rectifiers through which said direct current is passed to develop said direct voltage irrespective of whether said alternating cin'rent voltage is increasing or decreasing, means for developing-an alternating current voltage as a function of said rectified voltage when it exceeds a given value, means for varying said given value, and means for impressing said developed voltage upon said first voltage to form the controlling voltage. v

9. In a device for controlling and stabilizing an airplane equipped with airplane controls and with means for detecting deviation about one of its axes, comprising in combination means for expressing the magnitude of this deviation in terms of an alternating current voltage which is a function of said magnitude, means for detecting the rate of said deviation, and means for expressing said rate of deviation in terms of an alternatinfl current voltage which is a function of said rate,

means for amplifying such rate-expressing alter-' nating current voltage, means effective when the amplified rate-expressing voltage rises above a selected value to impress such amplified voltage on the first mentioned voltage, and'means for operating the airplane controls in accordance with the resultant voltage by stages in which the operating means overruns the required adjustment of the controls and then is reversed to approach the required adjustment.

10. In a device for controlling and stabilizing an aircraft equipped with a gyroscope. aircraft controls and a servomotor for operating said controls, a potentiometer controlled by said gyroscope, means for impressing an alternating current voltage across said potentiometer, said potentiometer including a center tap connected with said gyroscope to move in accordance with deviations in movement of said aircraft, a second potentiometer, means for impressing an alternating current voltage across said second potentiometer, said second potentiometer having a center tap controlled by said servomotor and being at the same potential as the first mentioned center tap when both center taps are in central position whereby deviation of the aircraft or movement of the servomotor will result in a difference in potential between the two taps, means for expressing this difference in terms of alternating current voltage to operate said servomotor, a third potentiometer connected between the first mentioned center tap and ground and having a third center tap, an electron tube having a grid connected to said third center tap, a cathode connected to ground through a condenser ands resistor in parallel and a plate, and a transformer having its primary connected to said plate and its secondary connected to means whereby an amplified alternating voltage will be impressed on the first mentioned voltage to operate said servomotor accordingly.

WALTER J. nansmaanam. DONALD E. PU'I'I.

REFERENCES crrEn The following references are of record in the file of this patent:

Number Name 1 Date 2,159,142 Fischer May 23, 1939 2,190,390 Thiry Feb. 13, 1940 2,310,954 Hodgman Feb. 16, 1943 2,317,383 Hull Apr. 2'1, 1948 I FOREIGN PA'I'ENTd Number Country 1 Date 260,260 Great Britain Z' Jan. 12, 1928 .o'rrma REFERENCES A. P. C. Application of A. Kronenberger. Serial No. 367,329; published May 25, 1943. Y 

